(1) Field of the Invention
The present invention relates to glass furnaces, especially end-fired furnaces with a recuperator or with regenerators. It relates more particularly to a process and a system for a glass furnace, especially an end-fired furnace provided with a recuperator which makes it possible to maintain the output efficiency of the glass furnace when there is a failure of the recuperator or of the regenerator and throughout the period of its blocking.
(2) Description of the Related Art
A glass furnace is usually supplied with energy through the intermediary, in particular, of so-called "air-fuel" ports into which a mixture of air and of fuel, such as natural gas, fuel oil or the like, is injected. These furnaces usually have at least one recuperator or regenerator which allows air to be preheated before it is injected into the ports. When the recuperator or regenerator of the furnace is defective or damaged, this furnace being normally heated with air ports, it is no longer possible to preheat sufficiently the air injected into these ports, and the energy consumption in the ports increases. Sometimes the energy contributed by the flame is no longer sufficient and cold spots are found on or in the bath of glass, which are highly detrimental to the quality of the glass produced and to the yield of the furnace.
The blocking of a recuperator in a furnace can demand several weeks or several months and during this period of blocking the production of glass in the furnace must be continued. The problem presented consists therefore in finding a means for replacing the function of the recuperator while employing, if possible, less energy than when the furnace is in normal operation with its recuperator.
Glass furnaces can be classified into various categories according, in general, to the type of glass manufactured and its use downstream of the melting and refining furnace. Glass furnaces for the manufacture of glass fibers generally comprise one or several recuperators which recover heat from the fumes by heat exchange with the air entering the furnace to feed the ports. These furnaces are generally furnaces in which the heating/melting of the glass charge takes place by virtue of a plurality of ports arranged in the side walls of the furnace.
Another type of furnace consists of so-called regenerative furnaces, which comprise at least two regenerators operating alternately in time:
Before being discharged to the atmosphere, the fumes pass through these regenerators and transfer part of the heat to refractory walls placed as a chicane in the regenerator. Every 20 to 30 minutes the ports and the regenerators are generally switched so that the regenerator which previously carried the hot fumes from the ports carries the cool air which feeds the ports and vice versa. This cool external air is thus heated in contact with the walls of the regenerator.
There are essentially two types of regenerative furnaces: those of the type with transverse ports which comprise series of ports in the side walls of the furnace with a regenerator facing each of them, and those of the end-fired type, which comprises two adjacent regenerators situated in the upstream part of the furnace (in relation to the flow of the glass charge), generally with one or more fuel injectors situated under the opening of each regenerator which opens into the furnace, with the result that the fumes from the flames produced by the burning of the fuel and of the preheated air are recovered in the other regenerator, and vice versa when the switching from one system of ports to the other takes place (every 20 to 30 minutes).
The present invention relates essentially to these latter regenerative furnaces of the end-fired type.
More details on glass furnaces can be found, for example, in the work entitled "Glass Furnaces--Design, Construction and Operation"--Society of Glass Technology--1987--by Wolfgang Trier.
Many documents in the art describe the use of oxygen-fuel ports in glass furnaces, either supplementing existing ports, or as a replacement for said ports.
For example, European Application EP-A-0,447,300 in the name of the Applicant Company, discloses a process for melting and refining in a glass furnace of end-fired type, in which at least one port of oxygen-fuel type is added in the glass refining zone, said port being of the type in which a so-called "pulsed" combustion is performed on the fuel or the oxidizer, so as to reduce the production of NOx in the fumes.
U.S. Pat. No. 5,116,399 describes a process for melting glass in an end-fired furnace which comprises only one oxygen-fuel additional port situated in the glass-refining zone in the middle of the front wall of the glass furnace, pointing toward the melt, so as to keep the unmelted material substantially on this side of the array of the bubblers which separate the melting zone from the refining zone. For this purpose the gases of the flame issuing from this additional port must have a speed of at least 100 m/s, which makes it a so-called "high-impulse" flame which has, in particular, a major disadvantage, when thrown forward at the limit of the melt, of causing the projection of unmelted glass particles.
Among the documents cited above and relating to regenerative glass furnaces of the end-fired type, none relates to the operation of such a furnace without regenerators, that is to say none concerns in particular the problem of the operation of this furnace during the blocking of the regenerators.
Similarly, the publicity document entitled "Glassman Europe '93--Presented at Glassman Europe '93 Lyon--France--Apr. 28, 1993" by G. B. Tuson, H. Kobayashi and E. J. Lawers is completely silent on this problem.
In particular, the problem which the inventors have had to solve, after having contemplated employing flames in which the oxidizer contains more oxygen than air does, and preferably more than 50 vol % of oxygen, consisted in keeping the flame loop-shaped, that is to say heating substantially the whole surface of the bath of glass: this loop shape is relatively easy to produce when air is employed, but this is found to be difficult when the oxidizer is pure or substantially pure oxygen (that is to say containing preferably more than 88 vol % of oxygen) and when the speed of injection of the oxygen or of the oxidizer gas remains low and generally does not exceed 60 m/s (for example in the case of oxygen supplied either from a stock vessel of liquid oxygen or from a low-pressure adsorption system of the VSA (Vacuum Switch Adsorption) type), but when the speed of the fuel gas is high (that is to say in particular when the ratio of the speeds of the fuel and of the oxidizer is higher than 1). The speed of the fuel gas, often higher than 100 m/s, then causes rapid oxidizer/fuel mixing and hence a shortening of the flame which then loses its loop shape. (The higher the speed of the fuel gas is than that of the oxidizer gas, the more the mixing of the two tends to take place rapidly).